This invention relates to structural panels that are fabricated from sheets of thermoformable polymer materials.
Thermoformed twin-sheet pallets are generally known to the prior art as providing a leg supported load platform for storing and transporting goods and material. The leg support distinctive to pallet construction is considered as essential to the convenient placement of pallet lifting or moving elements. Frequently, these lifting elements are the blades or tines of a fork truck.
The need to provide protected space under a pallet for lifting blade penetration presents distinctive design constraints for twin-sheet pallet fabricators. Panels of substantially uniform thickness between specially separated top and bottom sheets present other design and fabrication constraints. One highly desirable characteristic of a loading panel, which has no leg or floor rail elements, is substantially uniform bending strength about mutually normal polar movement axes within the panel plane.
Another desirable characteristic, common to any thermoformed twin-sheet article, is a rapid fabrication cooling rate. Generally, twin-sheet articles are formed in a vacuum mold having respective forming dies in each of two half shells. A first sheet of hot, plastic flow thermoformable polymer is gas sealed against one forming die and a vacuum drawn between the die form and the sheet. The uniformly distributed atmospheric pressure load on the "outer" face of the hot polymer sheet presses it into intimate contact with the die form surface.
With the first sheet formed, the vacuum is sustained against the first formed sheet while the mold is opened and a second unformed sheet of hot polymer is positioned with a gas seal against the other forming die. The mold is again closed to press the first sheet form into the second sheet as a vacuum is drawn between the second sheet and the other die form surfaces.
At the very heart of the twin-sheet structural strength is the moment separation space between the planes of the two sheets. However, consistent with traditional I-beam practice, the two panels must also be unitized by tying structure. It is the presence of such tying structure that creates closed cells in the void space between the two constituent sheets.
After the twin-sheet article is formed, it must remain in the mold until it cools sufficiently for self-support and form stability in the absence of mold confinement. Such retention of the article in the mold terminates mold production during the cooling interval. In some cases, the void space between the formed sheets is penetrated by a pressurized air discharge conduit to cool the article with an internal flush flow of cooling gas, usually air. However, if the internal volume of the article includes a significant percentage of closed cell volume, the flush flow of cooling air has a substantially reduced effect.
It is, therefore, an object of the present invention to provide a thermoformed twin-sheet structural panel having uniformly oriented bending strength.
Another object of the invention is a twin-sheet structural panel having a high rate of post-formation cooling.
Also an object of the invention is a twin-sheet panel adapted to a high rate of productivity.
A further object of the invention is a twin-sheet panel having substantially no closed cells within the internal volume.
An additional object of the invention is a twin-sheet panel having a substantially open and internally vented void volume.